Identification

Description

Valves are elliptic to linear-elliptic and relatively flat. The raphe (R) valve has a narrow axial area and a small circular or oval central area. The raphe is straight and filiform. The distal raphe ends are straight and expanded externally. The proximal raphe ends are straight externally and slightly expanded. Internally, the proximal raphe ends are deflected to opposite sides. Striae are radiate and interrupted by a hyaline ring positioned close to the valve margin. The areolae are usually well distinguished under LM, 15-25 in 10 µm along a stria. The valvocopula attached to raphe valve has conspicuous fimbriae. The rapheless (RL) valve has a linear to linear-lanceolate axial area and radiate striae. The external openings of areolae are narrow, long transapical slits. The internal openings are comparatively shortened and are oval, or nearly round. There are 8-20 areolae in 10 µm along a stria in the RL valve.

The concept of Cocconeis placentula presented here is in the broad sense (sensu lato). We group several varieties (and species), including the nominate variety C. placentula var. placentula, C. euglypta and C. lineata. The reason for presenting such a wide concept of this species is the lack of clearly established differentiating criteria between these taxa in monitoring programs. There has been a great deal of confusion because the RL valve is needed to determine the variety or species; the R valves of the varieties appear to be indistinguishable. In our experience, neither original descriptions of these taxa, nor the descriptions available in the floras are sufficient to separate them with any degree of certainty. Jahn et al. (2009) initiated a study of type materials of Cocconeis, including that of the nominate variety of C. placentula. Recent work (Romero and Jahn 2013) provided lectotypification of C. lineata and C. euglypta, yet further studies are necessary to determine whether and how individual species within C. placentula sensu lato may be distinguished using morphological characters.

Light micrograph of living cells of Cocconeis placentula attached to a glass slide. Cells of Achnanthidium are also present. Chloroplasts are visible within the silica cell walls.

Credit: Sarah Spaulding

Scannning electron micrograph of living cells of Cocconeis placentula attached to a rock. Note the mucilage pads present around a number frustules. The raphe valve is fixed to the substrate, while the rapheless valve faces up. Scale bar is equal to 50 µm.

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Responses

The 15 response plots show an environmental variable (x axis) against the relative abundance (y axis) of Cocconeis placentula sensu lato from all the stream reaches where it was present. Note that the relative abundance scale is the same on each plot. Explanation of each environmental variable and units are as follows:

ELEVATION = stream reach elevation (meters)STRAHLER = distribution plot of the Strahler Stream Order SLOPE = stream reach gradient (degrees)W1_HALL = an index that is a measure of streamside (riparian) human activity that ranges from 0 - 10, with a value of 0 indicating of minimal disturbance to a value of 10 indicating severe disturbance.PHSTVL = pH measured in a sealed syringe sample (pH units)log_COND = log concentration of specific conductivity (µS/cm)log_PTL = log concentration of total phosphorus (µg/L)log_NO3 = log concentration of nitrate (µeq/L)log_DOC = log concentration of dissolved organic carbon (mg/L)log_SIO2 = log concentration of silicon (mg/L)log_NA = log concentration of sodium (µeq/L)log_HCO3 = log concentration of the bicarbonate ion (µeq/L)EMBED = percent of the stream substrate that is embedded by sand and fine sedimentlog_TURBIDITY = log of turbidity, a measure of cloudiness of water, in nephelometric turbidity units (NTU).DISTOT = an index of total human disturbance in the watershed that ranges from 1 - 100, with a value of 0 indicating of minimal disturbance to a value of 100 indicating severe disturbance.

Cocconeis placentula sensu lato

Ehrenb. 1838

Valves relatively flat, elliptic to linear-elliptic

Raphe valve with small central area

Striae on raphe valve interrupted by hyaline ring

Striae radiate

Valves are elliptic to linear-elliptic and relatively flat. The raphe valve has a narrow axial area and a small circular or oval central area. The raphe is straight and filiform. Striae are radiate in both raphe and rapheless valves. The striae on the raphe valve are interrupted by a hyaline ring positioned close to valve margin...